The invention relates to an improved composite abrasive compact having high thermal stability and method of manufacture.
It is well known to consolidate a mass of natural or synthetic diamond particles into a compact by means of a high pressure, high temperature (HP/HT) press and a suitable diamond-forming sintering aid. Apparatus and techniques to accomplish the necessary sintering of the diamond particles are disclosed in U.S. Pat. Nos. 2,941,248 Hall and 3,141,746 DeLai.
As a means of increasing the strength of a polycrystalline diamond compact and providing a convenient means for mounting as a cutting or abrading surface it has been proposed to attach the compact to a stiff carbide substrate with a strong adhesive bond. U.S. Pat. No. 3,745,623 Wentorf et al teaches sintering of the diamond mass in conjunction with tungsten carbide to produce a composite compact in which the diamond particles are bonded directly to each other and to the cemented carbide substrate.
The DeLai and Wentorf et al processes employ a solvent-catalyst sintering aid, preferably cobalt, to facilitate the particle-to-particle bonding of the diamond during the sintering in the HP/HT press. The cobalt is either added to the diamond mass (DeLai) or derived from the tungsten carbide where it is employed as a cementing agent (Wentorf et al). Diamond and/or composite compacts manufactured in accordance with the teachings of DeLai and Wentorf et al have been limited to low-temperature applications since they thermally degrade at temperatures above approximately 700.degree. C. The thermal degradation results in accelerated wear when such compacts are employed in high-temperature applications such as in rock bits drilling formations with compressive strengths above 20,000 psi.
Thermal degradation of sintered diamond occurs when solvent-catalyst materials, such as cobalt, for example, are used as the sintering agent in the manufacturing process. The same solvent-catalyst action is relied upon using cobalt or other solvent-catalysts to accomplish the cementing action of sintering of diamond and to catalyze the formation of diamonds. This ability, while most pronounced at elevated temperatures, is retained to a lesser degree at temperatures below the melting point of cobalt. Therefore, the cobalt, present in the conventional sintered diamond process dissolves the diamond at the temperatures generated in the drilling of harder rock and at atmosphere pressure the carbon precipitates as graphite at the diamond grain boundaries within the compact. Graphitization is greatly accelerated at and above the melting point of cobalt with consequent degradation of the compact until catastrophic failure when the diamond loses its ability to cut or drill rock.
To avoid the problem of thermal degradation and permit the effective use of polycrystalline diamond compacts at temperatures above 700.degree. C. it has been proposed to leach the metallic second phase from the compact leaving essentially diamond. See U.S. Pat. Nos. 4,224,380 Bovenkerk et al and 4,104,344 Pope et al. However, the diamond compacts produced in accordance with this proposal can not be fully consolidated and have a typical density of approximately 92%. In addition, no practical method has been proposed for adhering a supporting substrate to the compact. It was originally believed that the compact could be leached after it was bonded to the substrate without weakening the strength of the bond, but this has not proved to be feasible.
It has been generally recognized that a diamond compact formed with silicon or other nonsolvent-catalyst as a sintering aid would have enhanced abrasive characteristics plus an extended use at high temperatures above 700.degree. C. In the HP/HT press the silicon is converted to silicon carbide which has excellent abrasive characteristics of its own and which will not catalyze the back-conversion of diamond to graphite. When silicon is used directly in sintering unbonded diamond particles, silicon is converted to silicon carbide and the sintering process is stalled before it is completed and the wear characteristics of the resultant compact are too less abrasion resistant required for drilling harder rock. In addition, because of the apparent chemical reaction between silicon and the cobalt binder in cemented tungsten carbide it has not been possible to bond a tungstencarbide substrate directly to the compact. See U.S. Pat. No. 4,308,471 Lee et al.